1. Field of the Invention
The present invention relates to a coating resin composition which contains as a crosslinking agent a melamine-formaldehyde resin which permits a paint to cure at low temperatures, permits a paint to form a coating film flexible enough for the fabrication of the coated substrate, gives off only a small amount of formaline at the time of paint application, and makes it possible to increase the solids content in a paint.
2. Description of the Prior Art
It is known that heat-curing paints include resin compositions composed of an alkyd resin containing carboxyl groups and hydroxyl groups, polyester resin, or acrylic resin, and a melamine-formaldehyde resin (abbreviated as melamine resin hereinafter) obtained by the reaction of melamine with formaldehyde, followed by modification with an alcohol. The melamine resin for this use is usually methyl etherified methylol melamine resin, butyl etherified methylol melamine resin, or mixed alkyl etherified methylol melamine resin (see Japanese Patent Publication No. 40187/1982).
The heat-curing paints as mentioned above, which find use in the fields of precoat metal, automobiles, and home electric appliances, are recently required to meet the following requirements to rationalize coating operations and minimize enironmental pollution.
(1) The baking temperature should be low so as to save energy, reduce costs, and increase efficiency.
(2) The coating film should have sufficient flexibility for the fabrication of the coated substrate. (Galvanized steel sheets and aluminum sheets are factory-finished into coated panels called precoated metal, colored galvanized steel sheet, or colored aluminum sheet. They undergo fabrication such as bending and punching.)
(3) The paint should not give off at the time of application more solvent and formaldehyde than permitted for environmental pollution control. In addition, it should have a high level of solids content.
To meet these requirements, many attempts have so far been made. They include the use of a methylol-rich melamine resin or an acid catalyst where the lowering of baking temperatures is required.
The disadvantage in the former case is that the cured coating film lacks flexibility and fabricability because methylol-rich melamine resin is a polynuclear compound formed by condensation of many melamine nuclei. In addition, a methylol-rich melamine resin is not readily available in the form of high solids content and hence it is not suitable to increase the solids content in a paint. Moreover, a paint containing a methylol-rich melamine resin gives off a large amount of formaldehyde and organic solvents at the time of application.
In the case where an acid catalyst is used, the melamine resin is usually alkyl etherified methylol melamine resin, which is composed mostly of mononuclear molecules, containing a small amount of methylol groups and imino groups. It is available in the form of high solids content. A paint containing it has a high solids content, gives off only a small amount of organic solvents and formaldehyde at the time of application, and provides a coating film flexible enough for the fabrication of the coated substrate. A disadvantage of using an acid catalyst is that it is difficult to balance between the properties of the coating film and the type and amount of the acid catalyst used. Another disadvantage is that the residual acid catalyst in the coating film impairs the water resistance and chemical resistance of the coating film and discolors the coating film. In addition, an acid catalyst in a paint causes the discoloration of pigments, especially basic pigments, and impairs the storage stability of the paint.
There is another type of coating resin composition which provides the coating film which is suitable for the fabrication of the coated substrate and has improved weather resistance and durability. It is composed of a newly developed resin and a miscible methyl etherified methylol melamine resin as a crosslinking agent. The former includes, for example, oil-free alkyd resin or oil-free polyester resin (which contains no or only a little natural oil), high-molecular weight polyester resin, silicopolyester resin, silicoacrylic resin, fluoropolyester resin and fluoroacrylic resin. This coating resin composition, however, has a drawback of giving rise to foams, pinhole, fish-eyes, craters, etc. which impair the appearance of the coating film and the commercial value of the finished products. Presumably this drawback is attributable to the fact that the melamine resin is composed mainly of mononuclear molecules having a low molecular weight.
The requirement that environmental pollution be minimized is being met by revamping the existing coating facilities so that organic solvents and formaldehyde can be recovered and burnt. Needless to say, an ideal solution to this problem is to increase the solids content and lower the formaldehyde content in a paint.
As mentioned above, there have been proposed various kinds of melamine resins to meet the requirements of paints and coating film. However, they are not satisfactory yet. Among them butyl etherified methylol melamine resin is usually composed of polynuclear molecules. Consequently, a paint containing it does not provide a coating film having good flexibility and fabricability. In addition, this melamine resin is not readily available in the form of high solids content so long as it has a viscosity that permits normal handling, and hence it is not suitable to increase the solids content of a paint. A paint containing it gives off a large amount of organic solvents and formaldehyde at the time of application. These disadvantages may be circumvented if a proper production condition is selected so that the condensation of melamine nuclei does not take place and the resulting melamine resin is composed mainly of mononuclear molecules as in methyl etherified methylol melamine resin. However, the thus produced melamine resin requires a high crosslinking temperature and does not satisfy the requirement that the paint should have a low curing temperature.
Another one among the melamine resins is methyl etherified methylol melamine resin. Having a low molecular weight and being composed mainly of mononuclear molecules in most cases, it contributes towards increasing the solids content in a paint and reducing the amount of organic solvents and formaldehyde given off from a paint at the time of application. In addition, it helps to form a coating film superior in flexibility and fabricability because it does not undergo self-condensation appreciably. Moreover, it provides a more glossy coating film than butyl etherified methylol melamine resin. However, it has a drawback of giving rise to foams, pinholes, fish-eyes, craters, etc.
In contrast with the above-mentioned methyl etherified methylol melamine resin and butyl etherified methylol melamine resin, the mixed alkyl etherified methylol melamine resin has many advantages. It is readily available in the form of high solids content, and hence it contributes towards increasing the solids content in a paint and reducing the amount of organic solvents and formaldehyde given off at the time of application. In addition, it provides a coating film superior in flexibility and fabricability. The conventional one, however, is not satisfactory in that it does not permit the paint to be cured at a sufficiently low temperature and that it still gives off an untolerable amount of formalin at the time of application. With the melamine nuclei having a low average molecular weight, the melamine resin gives rise to foams, pinholes, fish-eyes, craters, etc. and does not contribute towards lowering the baking temperature of the paint.
In the case where a methyl etherified methylol melamine resin and a butyl etherified methylol melamine resin are used in combination with each other, the result is better than would be obtained when they are used individually; but it is not quite satisfactory.
In order to eliminate the above-mentioned disadvantages, the present inventors carried out a series of researches, which led to the finding that a coating resin composition containing a specific melamine resin specified below as a crosslinking agent cures at a low baking temperature, gives off only a small amount of organic solvents and formalin at the time of application, and provides a coating film which is free of foams, pinholes, fish-eyes, craters, etc. and superior in flexibility and fabricability. The melamine resin is characterized by that the melamine nucleus has residual imino groups after the reaction with formaldehyde, the formaldehyde bonded to the melamine nucleus is alkoxylated with two or more kinds of alcohols having different numbers of carbon atoms, and the molecule of the melamine resin has an average number of melamine nuclei in a certain range. It was also found that this melamine resin contributes towards increasing the solids content of the paint containing it. The present invention was completed on the basis of these findings.